Petroleum refiners usually employ hydrocracking process to produce desirable products such as turbine fuel, diesel fuel and other middle distillate products in the presence of a suitable hydrocracking catalyst. In recent years, since the vacuum gas oil becomes heavier in density and worse in quality, the hydrocracking catalysts are required to possess high activity, selectivity and stability for producing middle distillates from vacuum gas oil and the like, and especially to possess high resistance to various catalytic poison, in particular nitrogenous matter present in the feed material. Unfortunately, the prior art hydrocracking catalysts are suitable for processing a feed material with a nitrogenous matter level of no more than 10.times.10.sup.-4 % by weight (calculated as nitrogen), and when the nitrogen level of the feed material increases, the hydrocracking catalyst deactivates rapidly, in order to keep the productivity of the hydrocracking process constant, the operation temperature should be increased, which incurs more operation expense.
Each of U.S. Pat. Nos. 4,517,033, 4,517,074, 4,563,434, 4,576,711, 4,664,776, 4,672,048 and 4,762,813 disclosed a midbarrel hydrocracking catalyst, which comprises a molecular sieve, an amorphous aluminosilicate, an alumina and hydrogenation metal components, wherein said molecular sieve is LZ-10,LZ-210, modified LZ-210 or USY. Of the molecular sieves, LZ-10 is obtained by further hydrothermally treating USY, and LZ-210 is prepared by treating NH.sub.4 NaY molecular sieve with ammonium fluorosilicate in the presence of a buffer solution. However, when LZ-10 is used to prepare a midbarrel hydrocracking catalyst, the result catalyst achieves good selectivity to middle distillates but relatively low activity; while when LZ-210, modified LZ-210 or USY is used, the resulting catalyst possesses high activity but low selectivity to middle distillates.
It is known that the resistance of the hydrocracking catalyst containing a molecular sieve to nitrogenous matter can be improved by strengthening the resistance of said molecular sieve to nitrogenous matter. A known effective technique to strengthen the resistance of a molecular sieve to nitrogenous matter is treating NH.sub.4 NaY molecular sieve with fluorosilicate salts or fluorosilic acid in the presence of a buffer solution, but such a treating process provides molecular sieve with relatively low activity and resistance to nitrogenous matter.